A vast majority of the natural gas that billowed out of BP PLC's failed well in the Gulf this summer did not escape to the surface and atmosphere. Instead, the gas -- including its main component, methane -- remained trapped deep underwater, priming the bacterial response to the spill, according to research published online yesterday in Science.

Oil has long been the most visible component of the hydrocarbon rush that gripped the Gulf this summer, even when invisible, in the form of underwater mists of oiled water. Natural gas, billowing out from the Macondo well alongside oil at double the amount, often received scant attention from the public, press and government.

Fortunately, scientists were not so easily fixated on the crude.

In June, while the well resisted control, scientists led by David Valentine, a microbial geochemist at the University of California, Santa Barbara, took several hundred samples of natural gas at 31 sites in a large circle around Macondo, extending to a maximum of 8 miles from the spill's epicenter. Data from their cruise, sponsored by a grant from the National Science Foundation, amount to the first independent snapshot of the immediate, short-term response triggered underwater by an onslaught of gas.

The majority of the methane, they found, remained dissolved more than 2,600 feet underwater, and the gas likely accounted for two-thirds of all the microbial activity in the undersea plumes. Based on government and BP data, some 206,000 metric tons of methane, 35,700 tons of ethane and 28,400 tons of propane snaked out into the subsurface, they estimated.

"The concentrations in the deep plumes were typically more than 500 times the concentrations in the shallower water," Valentine said. "This is good evidence that the majority of methane was trapped and is consistent with most other works on the matter."

Scientists have long predicted that gas leaked from deepwater wells would fail to reach the atmosphere. (One prominent researcher, Samantha Joye at the University of Georgia, has stressed the importance of subsea methane for months, though her research has not yet been published.) The long, dark ascent gives gas plenty of time to dissolve into the ocean's cold stretches, even when it does not form into ice-like hydrate formations, Valentine said. And while this behavior had been seen in natural seeps, the BP spill provided the perfect laboratory to test these assumptions on a grand scale.

At several locations near the well, methane concentrations were so high that they equaled the amount of oxygen in the water, Valentine said, a disturbing balance. At the same time, however, the more volatile gas components were disappearing, their chemistry enticing to a host of deepwater bacteria, many closely related to known hydrocarbon degraders.

The study also highlights uncertainty as to which hydrocarbons, after propane and ethane, were next to be degraded in the undersea plumes, said Rich Camilli, the lead author of a report out of the Woods Hole Oceanographic Institution last month that first delineated the boundaries of large hydrocarbon plume that stretched southwest from the well.

"This suggests that most all of the microbial degradation that is happening there is just the natural gas being utilized, which suggests that there is proportionately less microbial degradation of the oil itself," Camilli said.

"This is where [this spill] is really unique," Camilli added. "For most oil spills, it's just oil, it's not natural gas, but there is so much natural gas that came out of this leak. It appears as though the microbes are just interested in the natural gas. So it suggests that the oil may persist longer than we would like."

Few bugs degrading methane

Unlike previous studies, which tracked the southwest plume and laid out the bacterial response to that mist, Valentine's survey provides a snapshot of a limited radius and does not speak to the plume as it grew more mature farther from the well. Like past studies, his report makes no claim on the current state of the Gulf's waters, nor does it attempt to quantify the ecological impact methane could have on the ocean's wildlife.

Simpler than crude oil's stew, natural gas is a mix of several small chemicals, each assembled out of hydrogen and carbon. Sturdy methane is the most abundant gas, accompanied by ethane, propane and butane, all more complex, larger and unstable hydrocarbons. Unlike methane, these gases can be eaten up by a wide variety of bacteria.

"Put something like propane or butane into a natural environment and it tends to be consumed very quickly," Valentine said.